This invention relates to a control system for a nuclear reactor. More specifically, this invention relates to a method and a system which enables an accelerated power reduction in response to incidents which require a sudden step reduction in reactor power without total reactor shutdown.
The modern nuclear power reactor is an intricate combination of many complex interrelated components and systems. During normal operation of the power reactor, it is not uncommon for undesirable events, such as failure of one of the components or systems, to occur, which, if neglected, would seriously jeopardize the safety and/or operability of the nuclear power plant. As a result, control systems for controlling the reactor system must be developed to respond to these events. The power producing cores of nuclear reactors are normally controlled in one of two ways: (1) the slow advance of individual control rods or groups of control rods into or out of the core to provide power manuvering control of the reactor, or (2) rapid insertion of control rods into the core to accomplish an emergency shutdown of the reactor. This second mode of control is called reactor "scram" or control rod "scram." Even though many of the possible undesirable events which can occur during normal operation require at most only a rapid reduction in power level, the conventional response to such undesirable events has been to shut down the reactor as rapidly as possible by scramming the control rods into the core. The main reason behind this excessive and sometimes unnecessary response is that the control rods cannot ordinarily be driven fast enough, into the core, even at their maximum speed of insertion, to satisfy the accelerated power reduction demand. A full scram on the other hand cannot be employed without completely shutting down the reactor. Full reactor scram is not ordinarily desirable since reactor scram is disruptive of the smooth operation of an electrical supply system and can give rise to a number of undesirable events. In the first place, the sudden change in reactivity imposed by rapidly operated control elements applies a thermal shock throughout the reactor system. This thermal shock is felt mostly in the fuel elements of the reactor which is the location where it is least desired. In the second place, an undesirable disturbance is applied to the electrical grid network to which the nuclear power plant is connected. In the third place, a complete shutdown may subsequently cause a delayed start-up of the power reactor due to xenon poisoning of the reactor core.
It is not uncommon that the necessity for an immediate reduction of power falls short of a requirement for the complete shutdown of the reactor. In such cases, partial reduction of the power level is permissible as long as the power reduction causes the reactor to assume a safe condition. Thus, if an accelerated partial power reduction can be accomplished, complete shutdown of the reactor can often be avoided. In this event the reactor operators may be able to diagnose and correct the original trouble thereby enabling the immediate return to full reactor power with a minimum of the undesirable events described above. Thus, in order to increase reactor availability, there is a need to provide a system which has the ability to respond to certain incidents, such as loss of one out of two steam generator feedwater pumps, by rapidly reducing power without complete shutdown of the reactor.